FIG. 1 shows a plot of the concentration of known proteins in cells versus the total number of known proteins (Archakov, A. I., Y. D. Ivanov, A. V. Lisitsa, and V. G. Zgoda, Afm Fishing Nanotechnology Is the Way to Reverse the Avogadro Number in Proteomics. Proteomics, 2007. 7: p. 4-9). Extrapolation of the known data to low concentrations suggests that there are many more unknown proteins present at concentrations well below the current detection limit. This limit is set by the smallest dissociation constant of affinity reagents used to collect proteins from cell extract or serum, and these are typically nM at best. In order to collect proteins at much smaller concentrations, large amounts of sample are generally required, together with a multitude of affinity reagents, so that binding by a very small fraction (e.g., when the concentration of sample is much less than Kd) provides a useable amount of sample.
Digital detection may be used to individually count captured molecules resulting in increased sensitivity. This has been demonstrated by Mok et al. (Mok, J., M. N. Mindrinos, R. W. Davis, M. Javanmard, Digital Microfluidic Assay for Protein Detection. Proc Natl Acad Sci USA, 2014. 111: p. 1323998111) via a “lab-on-a-chip” in which captured analytes are each tethered to a bead, where each bead is subsequently detected individually via a corresponding electrical signal in a narrow channel.